A wide variety of food products, such as pasta, beans, corn, peas, and other vegetables and fruit, are processed by blanching prior to being frozen, packaged and shipped. Generally, there are two types of industrial blanchers: the steam belt blancher, as exemplified by U.S. Pat. No. 4,942,810, issued Jul. 24, 1990 to Zittel, et al., and the rotary blancher, examples of which are disclosed in U.S. Pat. No. 5,146,841, issued Sep. 15, 1992 to Zittel, and U.S. Pat. No. 5,632,195, issued May 27, 1997 to Zittel. The steam belt blancher uses a conveyor belt to move food products through a substantially steam-tight chamber to subject the food product to a continuous, controlled temperature treatment in steam. Although the steam belt blancher has the advantage of using only steam, which is known to be a more efficient heat transfer medium than hot water, it suffers from having a great many moving parts, and is expensive and relatively slow in operation.
Conventional prior art rotary blanchers typically use hot water, or a combination of hot water injected with steam as the heat transfer medium. However, because rotary blanchers churn and tumble the food product through a turbulent hot water bath, they are capable of processing a larger volume of food product at greater speed than a comparably sized steam belt blancher. In addition, rotary blanchers are often further favored over steam belt blanchers because rotary blanchers are less expensive and can be more reliable because they have fewer moving parts.
In a rotary blancher, food product is introduced into an inlet end of an elongate cylindrically shaped drum, which is rotatably mounted in a generally cylindrical, open-top tank. The tank is fitted with a cover for enclosing the drum and can be opened for maintenance and cleaning. The drum has a helical auger running lengthwise through the drum that rotates during operation for transporting the food product from the drum inlet opening on one end to an outlet or discharge opening at the other end. The drum cylinder has sidewalls that are perforated to contain the food product in the drum while allowing the heat transfer medium to come in contact with the food product as it is transported by the auger through the drum. As the food product moves from the inlet end of the tank to the outlet end, the time it remains in the tank, its residency time, is controlled to ensure that the food product is blanched or cooked properly.
While rotary blanchers have been improved in many ways, it has been believed to be heretofore impractical to blanch or cook only using steam because steam leakage would be substantial and render it too costly. Prior art rotary blanchers have been at best only partially steam-tight. It is known that at least one prior art rotary blancher has a spaced apart pair of steam-tight water seals located where the tank and cover meet with each seal extending longitudinally the length of the tank. Each water seal consists of a trough carried by the tank that is partially filled with water for receiving one of the longitudinal edges of the tank cover.
Such seals are effective at preventing the escape of steam along the longitudinal sides of the tank. However, significant amounts of steam still can escape along the seam where the cover and tank meet at both ends of the blancher where there are no seals. In addition, steam can escape through the openings for the drum journals in the tank and tank cover endwalls, since there is clearance around the drum journals to allow the drum to rotate freely. Steam can also escape from the drum inlet and outlet openings where food product is introduced and discharged since both are open to the atmosphere.
To prevent moisture from escaping from inside the blancher, some atmosphere within the blancher is continuously vented to help create a negative pressure within the blancher. For example, for a blancher having a diameter of 5 feet and a length of 24 feet, about 1,500 cubic feet per minute/hour of atmosphere would typically be evacuated from inside the blancher during operation. Despite this, some moisture and steam still leak from the blancher. Moreover, while this rate of venting is suitable to safely minimize increasing the humidity of the plant in which the blancher is operating using either water or a combination of water and steam, it is not suitable to prevent or suitably minimize steam leakage where such a rotary blancher is using only steam as the heat transfer medium.
While a small amount of steam leakage does not present a significant problem in the case of conventional hot water operation, as steam is used to a greater extent to achieve higher temperatures and more rapid heat transfer, steam leakage becomes a significant problem. Loss of steam, of course, will result in loss of energy. One of the major expenses involved in blanching food product is the cost of energy needed for generating the steam or heating the water used to blanch the food product. In addition, if steam is allowed to escape into the factory it can condense and drip onto food product which can result in its contamination and loss. Moreover, increased humidity in the factory caused by the escape of significant amounts of steam can contribute to worker fatigue, and will result in generally unpleasant working conditions, slippery and hazardous factory floors, and accelerated corrosion of food processing equipment within the factory. Minimizing the amount of steam that escapes from the blancher is thus highly desirable.
Rotary blanchers using only steam as a heat transfer medium are believed to be capable of blanching food product as much as 10-20% faster than prior art rotary hot water or combined water and steam blanchers. However, due to the problems noted above, it has not heretofore been commercially achievable.
What is needed, therefore, is a rotary blancher that is sufficiently steam-tightly sealed so as to enable the rotary blancher to heat, blanch or cook using only steam. What is also needed is a blancher having a vent that can be controlled to draw excess steam out of the blancher at an appropriate rate so it will not escape from the drum inlet and outlet. What is still further needed is a rotary blancher that is capable of operating using a wide variety of heat transfer media including (1) water, (2) another liquid heat transfer medium, (3) a combination of water and steam, (4) a combination of another liquid heat transfer medium and a heated vapor, (5) only a heated vapor of another heat transfer medium, or (6) steam.